As the demand for broadband service communications increases, wireless communication networks are decreasing in cost and increasing in speed. In these circumstances, high power amplifiers incorporated in base stations and mobile phones today are a key device for building future fast wireless communication networks, and increase of the efficiency of the high power amplifiers is demanded.
For example, OFDM (orthogonal Frequency Division Multiplexing) modulation used by WiMAX (Worldwide Interoperability for Microwave Access), which is next-generation wireless broadband technology, requests amplifiers that operate with high output power because of their high peak power.
When communication services as mentioned above are provided, it is important to enable high power amplifiers to operate with higher efficiency, that is, to enable high power amplifiers to output higher power level radio signals with lower power consumption.
Recently, composite high power amplifiers, which incorporate multiple high power amplifiers, have been developed. A composite high power amplifier designed to certain specifications generally has functionality, amplification capability and effectiveness superior to standard high power amplifiers. Composite high power amplifiers therefore are being widely used in wireless communication devices.
Existing techniques include: an OFDM modulator that eliminates nonlinear distortion, proposed in Japanese Laid-Open Patent Publication No. 2000-22659; a technique that changes distortion compensation data for AM-AM (amplitude-amplitude modulation) characteristics or AM-PM (amplitude-phase modulation) characteristics to reduce distortion component power, proposed in Japanese Laid-Open Patent Publication No. 2006-140645; a Doherty amplifier that incorporates N amplifiers, proposed in “A Mixed-Signal Approach Towards Linear and Efficient N-Way Doherty Amplifiers”, IEEE TRANSLATIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, MAY 2007; and a technique to minimize phase distortion of LINC (Linear amplification using nonlinear components) transmitters, proposed in “An Adaptive Digital Method of Imbalances Cancellation in LINC Transmitters”, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 54, NO. 3, MAY 2005.
High power amplifiers or composite high power amplifiers are sometimes caused to operate in a nonlinear region in order to achieve high efficiency. However, when they are operating in a nonlinear region, distortion is exacerbated. Therefore, predistortion, which is a type of distortion compensation, is performed during operation in a nonlinear region.
In a composite high power amplifier, which includes multiple amplifiers, cross leakage in which outputs from the individual amplifiers interfere with one another occurs.
No measures have been taken against cross leakage in conventional composite high power amplifiers and signals affected by cross leakage have been directly used for predistortion. Accordingly, the conventional composite high power amplifiers have a problem that distortion may not be accurately compensated for and efficient signal amplification may not be performed.